import { decodeJwt } from 'jose';

// Refetch this long before the token expires.
export const MARGIN_MS = 60_000;

// Floor for short-lived tokens, so a backend issuing them cannot cause a refetch loop.
export const MIN_DELAY_MS = 10_000;

// setTimeout overflows (fires immediately) above this delay.
const MAX_TIMEOUT_MS = 2 ** 31 - 1;

/**
 * Fetches a token, hands it to a callback, and refetches shortly before it expires.
 * @internal
 */
export class TokenRefresher {
  private timer?: ReturnType<typeof setTimeout>;
  // Bumped by stop() so a fetch already in flight can tell it was cancelled.
  private generation = 0;

  constructor(private readonly fetchToken: () => Promise<string>) {}

  start(
    onToken: (token: string) => void,
    onError: (error: Error) => void,
  ): void {
    this.stop();
    const generation = this.generation;
    // The executor turns a synchronous throw from fetchToken into a rejection.
    new Promise<string>((resolve) => resolve(this.fetchToken())).then(
      (token) => {
        if (generation !== this.generation) return;
        let exp: unknown;
        try {
          exp = decodeJwt(token).exp;
        } catch {
          // Unreadable token: deliver it but cannot schedule a refresh.
        }
        if (typeof exp !== 'number') {
          onToken(token);
          return;
        }
        if (exp * 1000 <= Date.now()) {
          onError(new Error('Fetched token has already expired'));
          return;
        }
        const delay = exp * 1000 - Date.now() - MARGIN_MS;
        // Arm before delivering, so a stop() called from onToken cancels it.
        this.timer = setTimeout(
          () => this.start(onToken, onError),
          Math.min(Math.max(delay, MIN_DELAY_MS), MAX_TIMEOUT_MS),
        );
        onToken(token);
      },
      (error: unknown) => {
        if (generation !== this.generation) return;
        onError(error instanceof Error ? error : new Error(String(error)));
      },
    );
  }

  stop(): void {
    this.generation++;
    clearTimeout(this.timer);
    this.timer = undefined;
  }
}